1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic printer, for example, and an image forming method.
2. Description of the Related Art
In an image forming apparatus, such as an electrophotographic printer, which forms an image on a sheet, sheets are separated one by one from a cassette containing a plurality of sheets and is conveyed to an image forming section (e.g., a photosensitive drum). An image formed by the image forming section is transferred onto each sheet via a transfer roller. Then, the sheet is fed to a fixing section (e.g., a fixing roller) and is subjected to pressurization and heat treatment. After the transferred image is fixed on the sheet, the sheet is discharged outside the image forming apparatus.
Here, to transfer an image onto a sheet at a proper position, it is necessary to convey the sheet straight in a conveying direction without causing skewing of the sheet to the image forming section.
Conventionally, there has been a mechanism which stacks a plurality of sheets contained in a cassette in parallel to the conveying direction with a size regulating plate or the like provided in the cassette. However, mechanical means such as a size regulating plate cannot sufficiently correct skewing of a sheet.
There has also been a mechanism which corrects skewing of a sheet by causing a sheet fed from a cassette to be abutted against a registration roller disposed just before an image forming section.
Although this mechanism can sufficiently correct the skewing of a sheet with the registration roller, since conveyance of each sheet is temporarily stopped at the position of the registration roller, the time required for image formation becomes longer. The mechanism is thus unsuitable for an electrophotographic printer, such as an on-demand printer, which requires high productivity.
Additionally, it is impossible to perform registration in a lateral direction which is orthogonal to a sheet conveying direction (transverse registration) only by causing a sheet to be abutted against the registration roller.
To cope with this, there has been proposed a technique for correcting skewing of a sheet without stopping conveyance of the sheet by providing, a skewing mechanism which causes a sheet to be abutted against a stopper member parallel to the conveying direction while conveying the sheet obliquely to the conveying direction at a position just before the image forming section (see, e.g., Japanese Laid-Open Patent Publication (Kokai) No. 8-188300).
In this proposed technique, since the stopper member causes a sheet to be always conveyed at the same position in the lateral direction, good transverse registration can be achieved.
A sheet is conveyed while one side edge of the sheet along the conveying direction is in contact with the stopper member of the skewing mechanism. For this reason, a shifting mechanism which moves a sheet in the sheet lateral direction with a roller has been proposed to allow transfer of an image at a desired position. There has also been proposed a mechanism which changes the position of a stopper member in accordance with sheet size.
The shifting mechanism or the like allows an image to be always transferred onto a sheet at, e.g., a center position in the lateral direction in an image forming section. It is thus possible to centralize worn parts of a sheet conveying roller in an image forming apparatus and reduce skewing of a sheet. Since a sheet can always be fed to any post-processing device such as a stapler or a folding machine at a center position thereof, centering accuracy can be improved.
However, with an improvement in centering accuracy, there occurs a situation where microscopic asperities (rough projections) on two side edges of each of sheets of the same size in the lateral direction damage a fixing roller when the sheets are continuously conveyed. When a sheet larger in the lateral direction than the sheets having damaged the fixing roller passes through the fixing roller, the damage in the fixing roller causes a density difference in a toner image on the larger sheet.
In such a case, damage in a fixing roller is caused by sheets with the same width continuously passing through the fixing roller at the same position. To solve this problem, there has been proposed a technique for changing a sheet conveying position in the axial direction of a roller for every predetermined number of sheets (see, e.g., Japanese Laid-Open Patent Publication (Kokai) No. 10-293512).
To reduce damage in a fixing roller, a position at which a sheet is conveyed needs to be shifted upstream of the fixing roller in the sheet conveying direction. The method of shifting a sheet between a position at which a toner image is to be transferred onto a sheet and a fixing roller is available for this case. However, the arrangement of a sheet shifting mechanism between a transfer position and the fixing roller requires an increase in apparatus size and leads to a cost increase.
There is also available a method in which a sheet is shifted at the position just before the image forming section, as in the technique disclosed in Japanese Laid-Open Patent Publication (Kokai) No. 8-188300 described above. To shift a conveying position for every sheet and achieve good transverse registration, the positional accuracy of a sheet shifting mechanism needs to be improved.
Although a first possible method for improving the positional accuracy of a sheet shifting mechanism is to reduce the tolerance of the mechanism to as close to zero as possible, the method leads to a cost increase. Another possible method is to reduce the drive step size of the sheet shifting mechanism. However, shifting of a conveying position in micro-steps for every sheet slows the operation of the sheet shifting mechanism and significantly reduces productivity.